The present invention is in the field of continuous steel casting.
In continuous casting of steel, molten metal is poured from a large vessel called a xe2x80x9cTundishxe2x80x9d into a water-cooled copper mold by using a submerged entry nozzle (xe2x80x9cSENxe2x80x9d). Steel begins to solidify as it comes in contact with the walls of the copper mold, the slab descending down continuously at the casting rate. The thickness in a slab caster mold typically is about 9 to 12 inches, whereas in a thin slab caster the thickness is only about 2 to 4 inches. The width of the slab is generally very large, typically 60 to 72 inches. A layer of mold flux is maintained at the free surface of the metal, which protects the hot metal from atmospheric oxidation and provides a thin lubricating layer between the descending slab and the mold walls.
Several fluid flow studies of slab caster molds have shown that the flow of molten metal in a mold has a large influence on the surface and subsurface quality of the resultant cast metal. The molten metal exiting the submerged entry nozzle is at an angle relative to the horizontal and impinges on the narrow wall. This results in the formation of upper and lower recirculating flows, which are schematically shown in a general flow pattern in FIG. 1. The upper recirculation causes a standing wave at the free surface. The height of the wave typically oscillates with time. This oscillating standing wave and associated turbulence at the free surface is considered to be the main reason for most of the defects in cast slabs made by this process.
Due to other physical factors (e.g., the slide gate and non-uniform nozzle blockage) and turbulence, the patterns on the two sides in the mold may not be symmetrical and may continuously change over time. The wave height depends upon the SEN submergence depth, as the wave is typically higher with shallow submergence. The wave height also depends on the port angle and opening area, as smaller angles and smaller area typically yield a higher wave height. Surface turbulence and standing waves are probably the most important factors affecting cast quality. The wave and recirculations oscillate from one side to another, adversely affecting the quality of the cast.
The flow is further biased due to the influence of the slide gate or preferential nozzle blockage. The biased flow increases the chances of mold flux slag entrainment. A larger jet angle downward from the horizontal helps in reducing the surface turbulence and wave height by pushing the impingement point to a lower depth in the mold. A lower impingement point, however, results in a thin solidified shell at the exit from the mold and associates itself with a danger of breakout. Another problem is that a deeper lower recirculation carries the inclusions down to much greater depth and affects the quality of cast metal.
Accordingly, it is an object of the present invention to provide an apparatus and method for continuous casting of metal that provides for a flatter and less turbulent free surface to provide effective flux flow while also allowing for more efficient removal of inclusions and allowing for potential reduction in the risk of break out. This is expected to reduce surface and subsurface defects in the cast slab associated with this surface wave and turbulence.
Although described with respect to the field of steel casting, it will be appreciated that similar advantages of surface wave damping, along with other advantages, may obtain in other applications of the present invention. Such advantages may become apparent to one of ordinary skill in the art in light of the present disclosure or through practice of the invention.
The invention includes a molten metal vessel system for casting molten metal and a method of providing a flow of molten metal for continuous casting. In general terms, the apparatus of the present invention includes a molten metal vessel system for casting molten metal, the system comprising: (a) a vessel containing a molten metal adapted to contain and dispense the molten metal for casting, the vessel having interior surfaces and the molten metal forming an upper surface; (b) a submerged entry nozzle extending below the upper surface; and (c) a flow modifier member disposed between at least one of the interior surfaces and the submerged entry nozzle. A surface flow modifier member may be used that is in sufficient proximity to the upper surface of the molten metal so as to impede the formation of waves in the upper surface of the molten metal. A submerged flow modifier member may also be used that completely submerged at some depth below the upper surface of the molten metal, and in sufficient proximity to the submerged entry nozzle to affect the upper and lower recirculating flow patterns and resulting turbulence and wave action caused thereby. The submerged flow modifier member also has the effect of attenuating standing waves at the surface of the molten metal in the continuous casting mold.
Preferably, the surface or submerged flow modifier member(s) are located on either side of the submerged entry nozzle, and a series of surface or submerged flow modifier member(s) may be used on either side of the submerged entry nozzle. The surface flow modifier member(s) typically extend into the molten metal, although they may be adapted to reside just above the free surface of the molten metal so as to impede the formation of waves in the upper metal surface. The submerged flow modifier member(s) are typically situated below the surface of the molten metal at a location to impede the upper and lower recirculating flows generated by the flow of molten metal from the submerged entry nozzle. Normally, the molten metal surface will bear a flux layer. In certain embodiments, at least a portion of the surface flow modifier member(s) or the supports therefor may extend through the flux layer and/or the molten metal. In other embodiments, the entirety of the submerged flow modifier member(s) may be located within the molten metal and below the flux layer.
In general, the invention is not limited to any geometry of the surface or submerged flow modifier member(s). For instance, in one embodiment, the surface flow modifier member(s) may be shaped so as to provide a relatively thin portion adapted to extend through the flux layer and a relatively wide portion adapted to extend into the molten metal. In another embodiment, the surface flow modifier member(s) comprise(s) a plurality of tines adapted to extend through the flux layer and a relatively wide portion adapted to extends into the molten metal. In still another embodiment, the surface flow modifier member(s) comprise(s) a lower portion being tapered away from the submerged entry nozzle and toward the interior surface, so as to be somewhat trapezoidal in shape. In alternate embodiments, wherein the flow modifier member(s) are submerged in the molten metal below the flux layer, various geometries may be employed to reduce the turbulence and wave production caused by the molten metal exiting the submerged entry nozzle. In such an embodiment, the submerged flow modifier member(s) may have, for example, a polygonal, trapezoidal, or conical geometry. It should be realized, however, that these geometries are merely examples of the various geometries that may acceptably impede turbulence and wave action according to the present invention, while allowing as much as practicable the free and uniform flow of flux on the free metal surface. As such, other geometries may also be successfully utilized for constructing a surface or submerged flow modifier member.
The surface flow modifier member may be supported in contact with or in functional proximity to the free metal surface by any appropriate mechanical means, such as through a bracket attached to the casting mold. Attachment to a structure other than the casting mold may also support the surface flow modifier member. In certain embodiments of the present invention, submerged flow modifier member(s) may be also be attached to the submerged entry nozzle. Materials and attachment protocols may be any of those appropriate for the handling of temperature resistant materials, such as refractory ceramics. It is preferred that the surface or submerged flow modifier member(s) not contact the interior mold surface to avoid disrupting both the solidification of metal and the the flow of flux into the casting mold.
Another aspect of the present invention is that the flow modifier member(s) or other turbulence and/or wave impedance means allows the submerged entry nozzle optionally to direct the flow of molten metal at an angle at or above the horizontal (rather than the typical downward angled flow). This feature helps to promote a more efficient elimination of inclusions during the casting process, and also reduces the risk of high temperature break-out in the freshly solidified layer as it emerges from the mold occasioned by the hot molten metal being directed too low and too near the downstream end of the casting mold.
The molten metal vessel system for casting molten metal of the invention also includes (a) a vessel containing a molten metal adapted to contain and dispense the molten metal for casting, the vessel having interior surfaces and the molten metal forming an upper surface; (b) a submerged entry nozzle extending below the upper surface; and (c) means for impeding the formation of waves in the upper surface of the molten metal.
The means for impedance of the formation of waves in the upper surface of the molten metal may be accomplished by application of mechanical force (such as in the form of the surface or submerged flow modifier member or equivalent mechanical arrangement or device), fluid force (such as in the form of a gaseous flow directed against the free metal surface), or through application of electromagnetic force (such as through use of electromagnetic actuators used for other purposes of controlling molten metal in the industry).
The present invention also includes a method of providing a flow of molten metal for continuous casting, the method comprising: (a) providing a vessel containing a molten metal adapted to contain and dispense the molten metal for casting, the vessel having interior surfaces and the molten metal forming an upper surface; (b) conducting a flow of molten metal below the upper surface of the molten metal while impeding the formation of waves in the upper surface of the molten metal; and (c) allowing the molten metal to exit the vessel so as to form a metal casting.
The impedance of the formation of waves in the upper surface of the molten metal may be accomplished by application of mechanical device, fluid force or electromagnetic force as described above.
The submerged entry nozzle may direct a flow of molten metal at an angle at, above, or slightly below the horizontal, although it is preferred that the angle be slightly below the horizontal, (i.e. 1-20 degrees below the horizontal).
The present invention thus provides a simple method for reducing the turbulence and wave action typically inherent to the continuous metal casting process. In one embodiment, a piece of refractory or other temperature-resistant member (referred to generally as a xe2x80x9csurface flow modifierxe2x80x9d) is inserted, or otherwise engages the free surface, preferably from the top, and near and preferably on either side of the submerged entry nozzle. The surface flow modifier(s) then impede(s) the top recirculating flow and waves formed thereby, which in turn, significantly slows the surface velocity, and makes the free surface nearly flat. This is schematically shown in FIG. 2. In other embodiments of the present invention, a piece of refractory or other temperature-resistant member (referred to generally as a xe2x80x9csubmerged flow modifierxe2x80x9d) is located below the surface of the molten metal and provided to interrupt the normal recirculating flow pattern of molten metal from the submerged entry nozzle, thereby decreasing the magnitude of the standing waves generated thereby. Such an arrangement is illustrated in FIGS. 9 and 10. By employing the methods and apparatus of the present invention, defects caused by free surface waves or turbulence can be reduced or even practically eliminated.
Because the metal solidifies in contact with the mold wall, the surface or submerged flow modifier preferably should not touch the mold wall. Thus, it is typically necessary to maintain a gap between the surface or submerged flow modifier and the mold wall.
FIG. 3 shows a side view of the mold having a surface flow modifier as described above. This concept has been tested and proven to work in a small-scale water model. The shape, size and location of the surface flow modifier(s) will depend upon the particular system. Neither the surface or submerged flow modifier should slow down the flow to the extent that the metal freezes excessively near the free surface. A general three-dimensional schematic of the surface flow modifier system is shown in FIG. 3a. Since the surface flow modifier reduces the free surface turbulence, the flow becomes symmetrical on both sides of the submerged entry nozzle and significantly reduces the biased flow.
The method and apparatus of the present invention is expected to provide much better control over the continuous casting process and significantly improve the quality of the cast metal produced thereby.